GM:S 1.4 Sprite blank space causes border in blendmode

I do a lot of research at blendmode and shader these days, and I'm very glad to solve many problems, but there's a thing still not work fine.
If a mask is not full of image(I mean there's some empty space around it), then I will get some image with border finally, and the border is just same as the empty space. The only way to solve it is add some low alpha pixel at corner of the image, or make an inverse mask of the image. I guess gamemaker not draw the empty space at blendmode.
So I want to know how to fix it just use the origin sprite?
Thanks in advance for any help! It is very much appreciated!